Relationship between grass species richness and ecosystem stability in Kruger National Park, South Africa

The relationship between grass species richness and ecosystem stability was investigated in the Kruger National Park. A total of 135 489 individual grasses were identified from 189 sites spread across 19 485 km² of savanna biome, making this one of the largest studies of its kind. At each site, grass percentage abundance and standing crop were measured at one year intervals to provide an index of ecosystem function. A severe drought that affected the region between 1991 and 1993 provided a convenient perturbation. At the height of the drought, mean species richness declined by 12.7% while standing crop declined by 38.1%, from 3199 to 1979 kg ha^?1. Percentage grass abundance declined to 87.5% of its pre‐drought value. After the drought had passed species richness, standing crop and percentage abundance recovered to 92.1%, 113.8% and 92.8% of their pre‐perturbation values, respectively. Statistical analysis of these responses revealed that grass assemblages of low species richness were more resistant to drought than assemblages of high species richness. Species‐poor sites also showed better recovery from perturbation after the drought had passed. These findings suggest that ecosystem stability may be negatively related to grass species richness in South African savanna grasslands.     

Ucon-benzoyl dextran aqueous two-phase systems: protein purification with phase component recycling

Benzoyl dextran with a degree of substitution of 0.18 was synthesized by reacting dextran T500 with benzoyl chloride. A new type of aqueous two-phase system composed of benzoyl dextran as bottom phase polymer and the random copolymer of ethylene oxide and propylene oxide (Ucon 50-HB-5100) as top phase polymer has been formed. The phase diagram for the system Ucon 50-HB-5100-benzoyl dextran with a degree of substitution of 0.18 was determined at room temperature. This two-phase system has been used to purify 3-phosphoglycerate kinase from bakers' yeast. The top-phase polymer (Ucon) can be separated from target enzyme by increasing the temperature. The bottom-phase polymer (benzyol dextran) could be recovered by addition of salt. Yeast homogenate was partitioned in a primary Ucon 50-HB-5100-benzoyl dextran aqueous two-phase system. After phase separation the top phase was removed and temperature-induced phase separation was used for formation of a water phase and a Ucon-rich phase. The benzoyl dextran-enriched bottom phase from the primary system was diluted, and the polymer was separated from water by addition of Na₂SO₄.     

Impact of cell disruption and polymer recycling upon aqueous two-phase processes for protein recovery

A practical study is presented of the influence of cell debris and polymer recycling upon the operation of two-stage acqueous two-phase systems (ATPS) for the recovery of yeast bulk protein, pyruvate kinase and fumarase. Brewers' yeast was disrupted using one of two types of high-pressure homogenisers or a bead mill. The different cell debris suspensions were partitioned in a single PEG-phosphate ATPS extraction and the efficiency of solid-liquid separation was examined. A continuously operated two-stage ATPS process, using spray columns, is presented and practical problems of polymer recycling are discussed. Conclusions are drawn concerning the generic implementation and operational stability of ATPS in practical protein recoveries.     

Insights into the local residual entropy of proteins provided by NMR relaxation.

A simple model is used to illustrate the relationship between the dynamics measured by NMR relaxation methods and the local residual entropy of proteins. The expected local dynamic behavior of well-packed extended amino acid side chains are described by employing a one-dimensional vibrator that encapsulates both the spatial and temporal character of the motion. This model is then related to entropy and to the generalized order parameter of the popular "model-free" treatment often used in the analysis of NMR relaxation data. Simulations indicate that order parameters observed for the methyl symmetry axes in, for example, human ubiquitin correspond to significant local entropies. These observations have obvious significance for the issue of the physical basis of protein structure, dynamics, and stability.     

Characterization of a buried neutral histidine residue in Bacillus circulans xylanase: NMR assignments, pH titration, and hydrogen exchange.

Bacillus circulans xylanase contains two histidines, one of which (His 156) is solvent exposed, whereas the other (His 149) is buried within its hydrophobic core. His 149 is involved in a network of hydrogen bonds with an internal water and Ser 130, as well as a potential weak aromatic-aromatic interaction with Tyr 105. These three residues, and their network of interactions with the bound water, are conserved in four homologous xylanases. To probe the structural role played by His 149, NMR spectroscopy was used to characterize the histidines in BCX. Complete assignments of the 1H, 13C, and 15N resonances and tautomeric forms of the imidazole rings were obtained from two-dimensional heteronuclear correlation experiments. An unusual spectroscopic feature of BCX is a peak near 12 ppm arising from the nitrogen bonded 1H epsilon 2 of His 149. Due to its solvent inaccessibility and hydrogen bonding to an internal water molecule, the exchange rate of this proton (4.0 x 10(-5) s-1 at pH*7.04 and 30 degrees C) is retarded by > 10(6)-fold relative to an exposed histidine. The pKa of His 156 is unperturbed at approximately 6.5, as measured from the pH dependence of the 15N- and 1H-NMR spectra of BCX. In contrast, His 149 has a pKa < 2.3, existing in the neutral N epsilon 2H tautomeric state under all conditions examined. BCX unfolds at low pH and 30 degrees C, and thus His 149 is never protonated significantly in the context of the native enzyme. The structural importance of this buried histidine is confirmed by the destablizing effect of substituting a phenylalanine or glutamine at position 149 in BCX.     

双水相电泳分离蛋白质的研究

近几年来,随着生物技术的迅速发展,制备型电泳技术的研究得到了重视。然而由于技术上的原因,大规模的制备型电泳技术的研究还未能取得突破。阻碍电泳放大的一个主要问题是由于电加热作用而导致的热对流对电泳分离的破坏。为解决这一问题,人们提出了许多方法。例如,在太空的微重力环境下进行电泳,应力稳定自由流动电泳,循环等电聚焦和区带电泳,色谱电泳和等电膜等电聚焦等。这些方法在电泳放大上都取得了一定的进展,但各有其局限性。最近,Clark提出利用双水相的液液界面阻止热对流的设想,为开发大规模的制备型电泳技术开辟了一条新途径、Raghava Rao等在两种双水相体系上施加电场后成倍地缩短了分相时间。Levine和Bier采用U型管电泳装置研究了双水相体系中血红蛋白的电泳迁移率,观测到界面有阻滞作用。Clark在柱型电泳装置中进行了一组双水相萃取肌红蛋白的简单实验。在10mA的恒电流下电泳40min之后,肌红蛋白的分配系数为7.5,而当电场反向后,分配系数变为0.04,界面阻力并不显著,两者结论并不一致。     

Perturbations of the denatured state ensemble: modeling their effects on protein stability and folding kinetics.

By considering the denatured state of a protein as an ensemble of conformations with varying numbers of sequence-specific interactions, the effects on stability, folding kinetics, and aggregation of perturbing these interactions can be predicted from changes in the molecular partition function. From general considerations, the following conclusions are drawn: (1) A perturbation that enhances a native interaction in denatured state conformations always increases the stability of the native state. (2) A perturbation that promotes a non-native interaction in the denatured state always decreases the stability of the native state. (3) A change in the denatured state ensemble can alter the kinetics of aggregation and folding. (4) The loss (or increase) in stability accompanying two mutations, each of which lowers (or raises) the free energy of the denatured state, will be less than the sum of the effects of the single mutations, except in cases where both mutations affect the same set of partially folded conformations. By modeling the denatured state as the ensemble of all non-native conformations of hydrophobic-polar (HP) chains configured on a square lattice, it can be shown that the stabilization obtained from enhancement of native interactions derives in large measure from the avoidance of non-native interactions in the D state. In addition, the kinetic effects of fixing single native contacts in the denatured state or imposing linear gradients in the HH contact probabilities are found, for some sequences, to significantly enhance the efficiency of folding by a simple hydrophobic zippering algorithm. Again, the dominant mechanism appears to be avoidance of non-native interactions. These results suggest stabilization of native interactions and imposition of gradients in the stability of local structure are two plausible mechanisms involving the denatured state that could play a role in the evolution of protein folding and stability.     

静电作用与修饰铜锌超氧化物歧化酶的稳定性

铜锌超氧化物歧化酶(Cu, Zn-SOD)表面的赖氨酸经化学修饰后, 酶的稳定性显著提高. 赖氨酸被修饰后, 酶的电荷结构遂发生变化, 从而影响到酶分子电场. 使用FDPB方法(有限差分法求解Poission-Boltzman方程)计算了酶修饰前后的静电场变化, 以及对维持酶的结构稳定起重要作用的Cu, Zn配位结构的影响.结果表明, Cu, Zn配位体的两级离解常数在酶修饰后分别约下降10³, 10⁶.     

一类竞争扩散方程组在Neumann边条件下解的渐近行为

本文主要采用比较方法讨论了一类竞争扩散方程组在Neumann边条件下解的渐近行为,得到了解的稳定性区域．     

捕食者有无限时滞效应的两种群系统的全局渐近稳定性

本文用Liapunov泛函方法研究捕食者有无限时滞效应的捕食-被捕食系统的平衡状态的稳定性．文章提供了判定系统的平衡状态全局渐近稳定的简单条件,不要求积分核指数衰减.